The invention described herein relates generally to push buttons having nonlinear force/displacement characteristics, and more particularly to elastomeric push button return elements for providing enhanced tactile feedback.
Push buttons are widely used as switch actuators in keyboard terminals and a variety of other switch devices. A push button actuator typically employs some form of resilient element to bias the button toward its unactuated position. The simplest forms of resilient return elements typically provide a linear force/displacement characteristic. However, it is well known that a nonlinear characteristic may be advantageous in certain applications. Specifically, it may be desirable for the push button to exhibit a characteristic in which displacement force initially increases with displacement to a predetermined displacement value, and thereafter decreases with displacement. Such a characteristic may provide both more positive actuation of the switch or other device actuated by the push button, and tactive feedback indicating actuation of the switch or other device. These functions are desirable in keyboard and other applications.
Particularly in keyboards intended for high input rates over extended periods of time, the tactile characteristics of the push button keys become a major factor in mental and physical fatigue of the operator, and operator speed and accuracy. It it generally accepted that the optimum operating force for a high speed keyboard push button is in the range of 40 to 100 grams. It is further generally accepted that the length of the push button stroke before the breakover point in the force/displacement characteristic is an important factor in operator comfort. A push button stroke of 1.0 to 1.8 mm before the breakover point is regarded as the most desirable.
Other desirable characteristics or requirements of particularly a keyboard push button actuator include smooth and quiet operation, at least somewhat cushioned stops at the ends of push button travel, and long life. In addition, it is required that general purpose push button actuators be inexpensive, which translates into a requirement for a small parts count, parts made of inexpensive material, and ease of assembly. Finally, there are definite constraints on size. Keyboard designs have largely standardized on key spacing of 0.75 inches center to center, and the trend is toward increasingly low profile keyboard assemblies.
Aesthetically, it has been found desirable for keyboard keys to have a somewhat square configuration such that an array of keys substantially completely covers the underlying support panel, and to have sufficient apparent depth to conceal mechanisms other than the key tops mounted thereon. This has been accomplished with a key cap design having a substantially square top with a flaired skirt. In order to meet the low profile requirement, the push button mechanism, including any resilient return element therein, must fit within the skirt when the key cap is depressed. The fixed key spacing and the low profile requirements limit the space available for the push button mechanism, and consequently for the return element. As a result, both the mechanism and the return element must be exceptionally compact.
Return elements involving elastomeric dome configurations have been found to offer considerable potential in meeting the previously discussed requirements. A variety of elastomeric dome configurations for this purpose have been devised. One of the problems encountered with compact prior elastomeric dome designs is less than optimum deflection before the breakover point in the force/deflection characteristic. This problem is addressed in U.S. Pat. No. 4,390,765 issued to R. Sado, et al on June 28, 1983 in which is shown a dome design comprising an annular cylindrical portion and a hollow truncated cone portion arranged so that the junction between the two portions expands radially by a predetermined amount before the cone portion undergoes buckling deformation. Previous versions of a somewhat similar design are known from U.S. Pat. Nos. 3,478,857 issued to J. Linker on Nov. 18, 1969 and 3,767,022 issued to C. Olson on Oct. 23, 1973. U.S. Pat. No. 4,378,478 issued to R. Deeg on Mar. 29, 1983 and British patent application No. 2,112,577 published on July 20, 1983 also disclose elastomeric dome return elements having some general similarities to the previously described elements. The latter patent and application are of further interest in connection with the present invention for their showings of designs in which a radially extending flange surrounding and joined to the annular portion is formed with an annular groove at the location of the intersection between the flange and annular portion.
Although a variety of elastomeric dome return element designs have been devised in attempts to provide improved tactile characteristics for keyboard push buttons, none of the prior designs is optimally characterized in all respects. The applicants have devised a unique elastomeric dome return element design which provides specific improvements in the tactile characteristics achievable with a compact element, while continuing to meet the other important structural and operational requirements.